Using parallel ports for data acquisition and control

In process control and various instruments, real-time data acquisition and control can be completed by microcomputers. The information processed by the computer is always digital. The relevant parameters of the measured or measured object are often some continuously changing analog quantities, such as temperature, pressure, flow, speed, etc. Therefore, these analog quantities must be converted into digital quantities to be sent to the computer for processing. This process is analog-to-digital conversion (A/D). The digital quantity processed by the computer is converted into analog quantity to control the controlled object. This process is digital-to-analog conversion (D/A).
When using a PC for data acquisition, portable computers and notebooks have their own advantages. Portable computers and notebooks are light in weight, versatile, and easy to carry, meeting the requirements of data acquisition work anytime and anywhere. However, due to the lack of built-in ISA expansion slots required for data acquisition in portable computers and notebooks. Therefore, a parallel port or RS232 is required for data acquisition. If using a portable

Figure 1 General acquisition card structure

Figure 2 A dedicated data acquisition card

Figure 3 × Chamber pressure curve

The speed of data acquisition through the RS-232 serial port of a laptop or notebook will be greatly limited, so we use the parallel port (Standard Parallel Port) of a laptop or notebook for data acquisition. Parallel ports are divided into two types: ordinary and enhanced. Although the ordinary parallel port has a transmission rate of only 150KB/s, and because the data line is unidirectional, it cannot directly complete the data acquisition of the signal, and the enhanced parallel port has a transmission rate of up to 2MB/s, which can directly complete the data acquisition of the signal, but considering that models below 586 (except some late large 486) do not have enhanced parallel ports, in order to make the data acquisition card have stronger adaptability, the ordinary parallel port is still selected as the design object. [page]

1. Structure and working principle of universal data acquisition card
The structure of universal data acquisition card is shown in Figure 1, including weak signal amplification, channel conversion control, time base generator, timing controller, sampling/holding control, A/D conversion control, D/A conversion control, conversion drive circuit, data input and output interface control. Its working principle is that the measured quantity (such as temperature, pressure, flow, speed, etc.) is converted into an electrical signal by the corresponding sensor, and after passing through the weak signal amplifier (including voltage amplifier, current-voltage conversion, charge amplifier and low-pass filter), the weak signal is amplified to match the A/D conversion input voltage; the channel conversion control completes the task of switching multiple measured or controlled quantities in turn, and connecting with the analog-to-digital and digital-to-analog conversion circuits in time-sharing; the time base generator provides a clock signal or start signal for A/D and D/A conversion; the timing controller is used to control the correct progress of A/D conversion and D/A conversion; the sampling/holding circuit ensures that the input signal remains unchanged during the A/D conversion, which has a decisive influence on the accuracy of the A/D conversion; the conversion drive circuit completes the conversion and power amplification of the D/A conversion control output signal; the data reading and reading interface control circuit completes the connection with the parallel port.

Figure 5 Acceleration curve of ×× chamber

2. Application of general data acquisition card
Data acquisition card has various forms, and only two basic forms are introduced here.
2.1 Dedicated data acquisition card
Dedicated data acquisition card only collects a certain type or a certain type of measured signal, that is, it has a fixed sampling frequency and recording method, and usually only needs a few chips to implement it. Figure 2 shows a schematic diagram of a dedicated data acquisition card.
Figure 3 shows the × chamber pressure curve measured by the above device.
2.2 Storage test device
Figure 4 is a schematic diagram of the storage test system. Its working principle is: after reset, the storage test system continuously samples and converts the signal and writes it to the memory cyclically. When the trigger event arrives, the delay counter (module ⑥) is started to count according to the sampling frequency until the set value is reached, and the recording process is stopped. At this time, the memory address is the starting address of the record. Although this address is random, since we are only recording a process, we can completely get the required useful signal in the form of data.
The device composed of this type of circuit can effectively capture single transient signals, such as one-time instantaneous signals such as vehicle collision, projectile launch, and explosive explosion.
FIG5 shows the acceleration curve in the bore when the ×× projectile is fired, obtained by the above storage test system.